This invention relates to a temperature sensor, and more particularly to a temperature sensor comprising a holder and an ionic liquid contained therein.
Various types of temperature sensors are known including liquid-in-glass (LIG) thermometers, bimetallic thermometers, resistance thermometers, thermocouples, and radiometers. Depending upon the temperature to be measured, the required accuracy of the measurement, and other factors such as durability or cost, one type of temperature sensor may be preferable over another.
For instance, LIG thermometers are standard equipment at laboratories and surface weather stations. LIG thermometers have a fine glass bore and a fluid reservoir. Operation depends on the thermal expansion of the liquid contained in the glass envelope. The sensitivity of the LIG thermometer depends inversely on the diameter of the bore of the tube and on the relative expansion coefficients of the liquid and glass. The desired temperature range is the main criterion in the choice of the thermometric liquid.
The most common liquids used in LIG thermometers are the molecular liquids mercury (Hg) and ethanol. Hg LIG thermometers are inexpensive, durable, accurate and easily calibrated. Another advantage of the Hg LIG thermometer is its high temperature range (the upper operating temperature limit for a Hg LIG thermometer is about 350xc2x0 C.). Several disadvantages are that Hg is not be useful in low temperature situations because Hg freezes at about xe2x88x9239xc2x0 C., that Hg responds slowly in response to changes in temperature, and that Hg is highly volatile and toxic at low concentrations. Hg also poses an environmental hazard associated with the storage and disposal of broken Hg LIG thermometers.
For temperature measurements below xe2x88x9239xc2x0 C., LIG thermometers containing ethanol are commonly used. The ethanol LIG thermometer has a freezing point of about xe2x88x92110xc2x0 C. and a boiling point of about 78xc2x0 C. Ethanol has a faster response time and is less hazardous than Hg. However, fluid loss by evaporation is hard to avoid with ethanol and the upper operating temperature of 78xc2x0 C. limits the utility of ethanol LIG thermometers over a wide temperature range.
For many applications, the desirable temperature range is xe2x88x9270 to 370xc2x0 C. Therefore, two LIG thermometers must used in such a situation, an ethanol thermometer for low temperatures and a Hg thermometer for high temperatures.
In addition to the LIG thermometer, another commonly used temperature sensor is the bimetallic thermometer. Bimetallic thermometers are found in household central heating and air conditioning systems. Bimetallic thermometers rely on the differential thermal expansion of two metals bound together in a strip. They are cheap, durable, easily calibrated and can be used for thermographs. However, they require frequent calibration to maintain accuracy and they exhibit slow response times.
Yet another commonly used temperature sensor is the thermocouple. Thermocouples are used for in situ observations at locations wired to a computer network. Electrical and electronic thermometric devices deliver a rapid response, are durable and accurate over a broad temperature range. However, thermocouple and thermistor devices require expensive ancillary equipment and electronics to operate.
Still another device for temperature measurement is the radiometer. Radiometers are used for remote temperature observations. Radiometers permit measurement of temperature by detecting the absorption of emitted radiation. They are expensive specialist devices most commonly used for remote sensing on meteorological satellites.
The present invention provides one solution to the limitations of the currently available temperature sensors as is discussed in the disclosure that follows. Here, a temperature sensor comprised of a holder containing an ionic liquid delivers a wider working temperature range than most other temperature sensors, is economical, easy to calibrate, and the ionic liquid has low toxicity.
The present invention is directed to a temperature sensor comprised of an ionic liquid or mixtures thereof contained inside a holder.
In one embodiment, the temperature sensor is a holder using a thermometric liquid. The sensor comprises a thermometer having an elongated vessel and a reservoir for liquid. The thermometer, further, comprises a bore, within the elongated vessel, the bore being in liquid communication with the reservoir. In a preferred embodiment, a plurality of graduations are associated with the elongated vessel, such that the graduations can express a range of temperatures equivalent to about xe2x88x92100xc2x0 to about +400xc2x0 Centigrade. The liquid in the thermometer rises or falls, within the bore, depending on the temperature ambient to the thermometer.
In a preferred embodiment of the present invention the liquid is a thermometric ionic liquid that includes either a contrast or coloring medium or dye such that the level of the liquid in the bore can be more easily seen. As a result of the use of the ionic liquids of the present invention, the thermometer can measure temperatures in a range of about xe2x88x92100xc2x0 to about +400xc2x0 Centigrade.
In a temperature sensor of a preferred embodiment, the space within the bore is a vacuum. Further, as a result of the liquid used, the volume of liquid in the bore is dependent on temperature.
Also, the temperature sensor of the preferred embodiment can be created from a transparent material, such as glass or plastic.
The ionic liquid is made up of organic cations and either inorganic or organic anions or mixtures thereof.
A cation of an ionic liquid is preferably one whose structure corresponds to a formula selected from the group consisting of 
wherein R1 and R2 are independently hydrido, a C1-C6 alkyl group or a C1-C6 alkoxyalkyl group, and R3, R4, R5, R6, R7, R8 and R9 (R3-R9), when present, are independently a hydrido, a C1-C6 alkyl, a C1-C6 alkoxyalkyl group or a C1-C6 alkoxy group. It is to be noted that there are two iosmeric 1,2,3-triazoles. It is preferred that all R groups not required for cation formation be hydrido.
A cation that contains a single five-membered aromatic ring that is free of fusion to other ring structures is more preferred. Exemplary cations are illustrated below wherein R1, R2, and R3-R5, when present, are as defined before. A preferred organic cation is a 1-C1-C6-alkyl-3-methylimidazolium or a C1-C6 alkoxyalkyl-3-methylimidazolium cation. 
The anions of the ionic liquid can be hydrophilic or hydrophobic. An illustrative anion is selected from the group consisting of a halogen, pseudohalogen, a C1-C6 carboxylate, tetrafluoroborate, hexafluorophosphate, a polyfluoro C2-C6 carboxylate, bis(trifluoromethane-sulfonyl)imide, trifluoromethanesulfonate, and the like.
A preferred ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim] [Tf2N]). Another preferred ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim] [BF4]).
In another embodiment, a first ionic liquid is combined with a second ionic liquid in order to reduce viscosity and depress the solidification point. The second ionic liquid can be a colored ionic salt such as imidazolium tetrachlorometallate salts.
In yet another embodiment, a solvent (non-ionic liquid) can be added to the ionic liquid.